1. Field of the Invention
The present invention relates to an optical system for imaging or viewing distant objects based on reflected radiation and more particularly to an optical system for use with an imaging or viewing system which automatically compensates for bright spots which can overload and/or blind focal plane sensor arrays used in such optical systems.
2. Description of the Prior Art
Various systems are known for imaging and viewing distant objects that rely on reflected radiation from a distant object. Such imaging systems are known to be used, for example, in tracking systems for tracking moving objects, such as rockets and missiles. Both imaging and non-imaging tracking systems are known. Examples of imaging tracking systems are disclosed in commonly owned U.S. Pat. Nos. 5,939,706; 5,559,322; 5,900,620; 5,918,305; 5,939,706; 5,973,309 and 6,021,975. Examples of non-imaging type tracking system are disclosed in commonly owned U.S. Pat. Nos. 5,780,838; 5,780,839; 5,936,229; 5,955,724; 6,066,842 and 6,153,871.
Such imaging-type systems typically include an imaging device, such as an electronic camera, focal plane array, or the like, for detecting and tracking the position of a targeted moving object. While such imaging systems are effective in tracking moving objects, such systems are known to have limitations when used in combination with high power laser weaponry. For example, in such systems, the high power laser beam is known to interfere with the imaging system, potentially causing a loss of track of the targeted moving object. In particular, the reflected radiation from the laser hit spot is known to blind the imaging device or cause the automatic gain control to reduce the gain to compensate for the bright laser hit spot, thereby loosing the target image.
As such, non-imaging type tracking systems have been developed. Such non-imaging tracking systems are known to use a laser beam to seek and hold on to a glint, such as a cylindrical missile-roll axis. Unfortunately, such non-imaging systems can only be used with targets when such a glint is present.
To overcome this limitation, imaging tracking devices have been developed which can compensate for reflected radiation that tend to blind or saturate the imaging device. Examples of such systems are disclosed in U.S. Pat. Nos. 5,900,620 and 5,918,305. Such systems separate the reflected radiation from the target into two paths. The first path is the radiation from the laser-hit spot. The second path is the radiation from the target or image. A micromirror array is disposed in the optical path of the reflected radiation from the target.
The intensity of the radiation falling on the focal plane in the optical train following the micromirror array creates an electrical signal output for each pixel in the focal plane array. By construction, each pixel of the micromirror array corresponds to one or a group of detector pixels. If the received radiation on the detector focal plane exceeds a preset threshold, as determined from the electrical signal output, the reflectivity of the corresponding micromirror pixel is adjusted to reduce the irradiance on the affected portion of the focal plane array. As such, any bright spots in the reflected radiation from the target are then compensated by the micromirror array and reflected to an image plane.
The radiation reflected from the laser hit spot is directed to another image plane, at which a focal plane detector array is located. The location of the laser hit spot from the focal plane array is co-registered with the imaging array to form a self-referencing type imaging tracker device, which compensates for bright spots resulting from radiation reflected from a laser hit spot.
Commonly owned U.S. patent application, Ser. No. 09/687,754, filed on Oct. 13, 2000, relates to an anti-laser viewer. The ""754 patent application includes an optical system for separating radiation from a laser hit spot from the target information and providing separate imaging devices for both the target scene as well as the hit spot. The anti-laser viewer also includes a micromirror which compares the instant radiation at each pixel with a threshold level and automatically controls the reflectivity of the micromirror such that the intensity reflected by each pixel is within a desired threshold, thus compensating for bright spots in the reflected radiation from the target. Similar to the ""305 and ""620 patents, mentioned above, the anti-laser viewer relies on super-imposing the laser hit spot image on the target. Unfortunately, the anti-laser viewer, as well as the imaging type tracking systems disclosed in the ""620 and ""305 patents, require fairly complicated optics for compensating for bright spots in the reflected radiation from a distant object. In particular, these systems require a polarizing beam splitter, narrow band pass filters, a quarter wave plate and a micromirror assembly as well as multiple imaging planes. These multiple imaging planes must be co-registered to enable an image of the laser-hit spot to be superimposed on the image of the target and thus require relatively precise alignment. Thus, there is a need for a relatively simple device for protecting imaging devices from overload or saturation which does not require splitting incoming radiation into multiple optical paths and is relatively simple, has few components, and does not require precise alignment of components.
The present invention relates to an optical system for use with an imaging or viewing system, which automatically compensates for bright spots, which tend to overload or saturate imaging system, such as a focal plane array. The system can be used with imaging type tracking systems, viewers and various types of optical devices which heretofore have been unable to provide satisfactory performance due to saturation or overloading of an imaging device due to bright spots, such as laser radiation flares or sunlight. The system in accordance with the invention is configured such that the reflected radiation is imaged onto a first image plane without dividing the incoming radiation into two optical paths. A digital mirror device, for example, is disposed at the first image plane. The radiation level of each pixel in the image plane is compared with a fixed threshold on a pixel by pixel basis and used to generate a mirror drive signal that automatically reduces the reflectivity of the corresponding mirror pixel to compensate for bright spots.